The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles suspended in a solvent. This general type of display was first proposed in 1969. An EPD typically comprises a pair of opposed, spaced-apart and plate-like electrodes, with spacers predetermining a certain distance between the electrodes. At least one of the electrodes, typically on the viewing side, is transparent. For the passive type of EPDs, row and column electrodes on the top (the viewing side) and bottom plates respectively are needed to drive the displays. In contrast, an array of thin film transistors (TFT) on the bottom plate and a non-patterned transparent conductor plate on the top viewing substrate are required for the active matrix type EPDs. An electrophoretic fluid composed of a colored dielectric solvent and charged pigment particles dispersed therein is enclosed between the two electrodes.
When a voltage difference is imposed between the two electrodes, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus the color showing at the transparent plate may be determined by selectively charging the plates to be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of gray) due to an intermediate level of pigment particles attracted to the transparent plate may be obtained by controlling the plate charge through a range of voltages. No backlight is needed in this type of reflective EPD display, although it may be optionally added to improve the display viewability in the dark.
EPDs of different pixel or cell structures have been reported previously, for example, the partition-type EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148–1152 (1979)) and the microencapsulated EPD (as described in U.S. Pat. No. 5,961,804 and U.S. Pat. No. 5,930,026). Each of these has its own problems as noted below.
In a partition-type EPD, there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movements of the particles such as sedimentation. However, difficulties are encountered in the formation of the partitions, the process of filling the display with the fluid, enclosing the fluid in the display, and keeping the suspensions of different colors separated from each other. Even more difficult problems are encountered in the development of a roll to roll manufacturing process for such a partition type of displays.
The microencapsulated EPD has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a dispersion of charged pigment particles that visually contrast with the dielectric solvent. The microcapsules are typically prepared in an aqueous solution, and to achieve a useful contrast ratio, their mean particle size is relatively large (50–150 microns). The large microcapsule size results in a poor scratch resistance and a slow response time for a given voltage because a large gap between the two opposite electrodes is required for large capsules. Also, the hydrophilic shell of microcapsules prepared in an aqueous solution typically results in sensitivity to high moisture and temperature conditions. If the microcapsules are embedded in a large quantity of a polymer matrix to obviate these shortcomings, the use of the matrix results in an even slower response time and/or a lower contrast ratio. To improve the switching rate, a charge-controlling agent is often needed in this type of EPDs. However, the microencapsulation process in aqueous solution imposes a limitation on the type of charge-controlling agents that can be used. Other drawbacks associated with the microcapsule system include poor resolution and poor addressability for color applications.
U.S. Pat. No. 3,612,758 discloses another type of EPDs wherein the electrophoretic cells are formed from parallel line reservoirs containing charged pigment particles dispersed in a dielectric solvent. The channel-like electrophoretic cells are covered with, and in electric contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors.
The use of microchannels, microgrooves or microcolumns to form the EPD array still has problem of undesirable particle sedimentation or creaming along the column direction. In addition, the lack of a seamless, air-pocket free and continuous sealing process to enclose the electrophoretic fluid in between the two electrodes makes the roll-to-roll manufacturing extremely difficult.
An improved EPD technology and a roll-to-roll manufacturing process were recently disclosed in co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000, U.S. Ser. No. 09/759,212, filed on Jan. 11, 2001, U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 and U.S. Ser. No. 09/784,972, filed on Feb. 15, 2001, all of which are incorporated herein by reference. The improved EPD comprises closed cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent. This improved technology involving microcups allows high image quality in monochrome EPDs. A color display may also be manufactured by using a spatially adjacent array of small pixels formed of microcups filled with dyes of different colors (e.g., red, green or blue). However, there is a major deficiency in this type of system with only the traditional up/down switching mode, that is, the white light reflected from the “turned-off” colored pixels greatly reduces the color saturation of the “turned-on” color. More details in this regard are given in the following “Detailed Description” section.
While this latter deficiency may be remedied by an overlaid shutter device such as a polymer dispersed liquid crystal to switch the “off” pixels to the black state and keep the “on” pixels in the desired color state, the disadvantage of this approach is the high cost of the overlaid device, the high Dmin (which is the minimum optical density of the background) associated with the shutter device, and the complicated driving circuit design.
Alternatively, color displays of the normal up/down switching mode may be prepared by using color filters overlaid on the viewing side of the display. However, dark Dmin and lack of a high quality “white” state are the major problems for reflective color displays using color filters.
The “in-plane” switching concept was disclosed in E. Kishi, et al., “5.1: development of In-Plane EPD”, Canon Research Center, SID 00 Digest, pages 24–27 and Sally A. Swanson, et al., “5.2: High Performance EPDs”, IBM Almaden Research Center, SID 00 Digest, pages 29–31. In these prior art systems, the color effect is achieved by using a colored background with white or black particles. The disadvantage of these prior art systems is the lack of either high quality “white” or high quality “black” state. More details are also given in the “Detailed Description” section.
Thus, there is still a need for an improved EPD with high quality full color capability that can also be manufactured in an efficient manner, particularly by a roll-to-roll manufacturing process.